Rubber-covered plate cylinders are used to a great extent in the printing industry, and particularly in the flexographic printing industry. Flexographic printing plates are mounted on the rubber-covered plate cylinders. The major reasons for using rubber covered cylinders is (a) to enlarge the cylinder to different repeat circumferences, and (b) to print a seamless pattern thereon (see subsequent discussion). In flexographic printing, for example, a different size (circumference) of plate cylinder, i.e., for each different repeat circumference, is needed for each print pitch (length of the final image). A separate cylinder is also needed for each color, up to as many as eight different colors. These plate cylinders can cost $1,000 each. Moreover, a typical printing plant may have as many as 1500 cylinders in inventory which causes a significant storage problem. The number of plate cylinders can be reduced through the use of cylindrical print sleeves (as hereinafter described) having the requisite printing plates directly attached thereto.
The use of thick rubber coverings which are typically more than 0.375" up to about 1.00" thick is known. These rubber coverings can be fabricated directly onto the plate cylinder themselves, or can be fabricated onto a print sleeve which in turn is applied to a plate cylinder by known air expansion techniques. In this latter case, the printing plates are attached to the outer surface of the rubber covering. However, these thick rubber covered prior art plate cylinders or print sleeves have a number of problems. First, they require high temperature vulcanizing processes to produce these thick rubber coverings. These vulcanization processes can damage or deform the underlying plate cylinder or print sleeve which will ultimately impair the accuracy of the flexographic printing process. These processing problems are discussed in more detail below. The vulcanization process described above also requires long cure times.
Next, because of the above-described high thickness level (and the resulting high outer diameter), the rubber covered plate cylinder assembly cannot readily utilize standard plate cylinder sizes with standard print repeats and gear sizes. This will not only result in extraordinary additional direct costs in initial replacement of standard equipment, but it will put the end user in a position whereby all of the major components of the non-conventional flexographic printing system must be replaced with non-standard equipment. The density of the rubber covering must be very high (at least about 75 pounds per square foot) resulting in a rubber covered printing cylinder system that is very difficult to handle.
As discussed above, a major problem that exists with respect to rubber-covered printing cylinders, and particularly flexographic printing cylinders, resides in the process for applying the covering to the printing cylinder outer surface. Typically, printing cylinders are prepared by applying an uncured elastomeric material such as synthetic and natural rubber onto the outer surface of a cylinder followed by curing the material by vulcanization in situ. This vulcanization process typically employs the application of a combination of pressure and heat to the uncured elastomeric material in an autoclave for a period of time. After curing is completed, the cylinder with its hardened covering can be machined to the required size and finish. High curing temperatures during the vulcanization (in some cases above 300 degrees F.) can damage the cylinder. Other cylinders, such as composite cylinders, may suffer some form of physical degradation from the heat imparted when the rubber is cured. If the cylinders are made of metal, they can warp or loose temper during processing. Some cylinders are, due to their materials of construction, incapable of being vulcanized. Rubber covered sleeves for these cylinders are designed to operate at high thickness levels, namely, at 3/8" to 1".
Typically, flexographic printing plates are made from a flat substrate which is distorted to form a rounded surface to be mounted on round plate cylinders. This means that a seam is present. Regardless of how tight the butt joint is put together a seam still exists. By using a rubber covering on a plate cylinder of a smaller size, the correct or required print pitch can be obtained. Then, by engraving the image on a rubber covering, problems associated with round seams are avoided.
For quality printing, such as with photopolymer printing plates, a sticky backed thin cellular foam cushion member, generally made from an open cell foam material, is used between the printing plate and the cylinder. Typically, the printing plates are about 0.045 to 0.105" thick, and the cushion member is about 0.015" to 0.040" thick. This foam-backing system is used by approximately 90% of all printers in the United States who use this cushioning material.
The introduction of this cushion member between the printing plates and the plate cylinder causes a variation in the thickness dimension at various points along the outer surface of the rubber covering. This results in high or low spots in the outer printing surface. This thickness variation is relatively large, typically at least about 0.002 to 0.005". The prior art rubber coverings also have a limited impression range. Impression range, as hereinafter more fully defined, is the engagement distance to which the surface of the print substrate can be depressed by the print indicia during the printing operation without causing substantial visible reduction in print performance. For conventional high performance printing, the impression range will not exceed about 0.008".
Other problems associated with thin foam plate cushions are as follows:
a. Thin foams are not resistant to common printing solvents such as ethanol, n-propanol, isopropanol, ethyl acetate and n-propyl acetate. PA1 b. Thin foams are very labor intensive to mount and use. PA1 c. Thin foams are very fragile to handle, they are easy to abuse and damage. PA1 d. Thin foams have a limited useful life and cannot be remounted. PA1 e. Thin foams deliver relatively poor performance on heavy solid printed areas. PA1 f. Thin foams cannot be mounted without a seam. PA1 a. They have excellent resistant to all common flexographic solvents and inks, including solvents such as ethanol, n-propanol, isopropanol, ethyl acetate and n-propyl acetate. PA1 b. They are extremely durable and have very high tear-resistance, abrasion-resistance and tensile strength. PA1 c. They are transparent to the plate mounter thereby requiring little or no labor to mount. PA1 d. There are no thin foams to handle which are fragile, easy to abuse and damage, so that they have a unlimited useful life and can be readily mounted and remounted. Thin foams deliver relatively poor performance on heavy solid printed areas. PA1 e. They are totally seamless in design. PA1 1. Level the instrument. The handwheel in the frame behind the dial indicator will raise or lower the dial indicator. When the indicator is raised to its maximum height, a sample better than 1" thick may be measured. The knurled screw at the side is to lock the dial indicator in the desired position. PA1 Total Compression =X-M PA1 Recovery =X-R PA1 Compression Set =X-Y PA1 M=Thickness of sample under maximum load. PA1 R=Thickness of sample after maximum load has been removed. PA1 X=Original thickness of sample PA1 Y=Recovered thickness of sample
Another issue is the fact that in conventional formation techniques for the above printing cylinders and sleeves, the exact hardness is difficult to obtain with long term repeatability. Therefore, such processes for producing rubber-covered cylinders or sleeves can be quite expensive. However, if a rubber-covered cylinder or sleeves can be reproducibly manufactured at a predetermined hardness level, the printing plates can be directly mounted onto the rubber covering without the need for cushioning. Cushioning is a problem because it cannot be assembled in a uniform thickness. Further, it adds significant expense to the cost of the cylinder or sleeves, and it can also degrade before the plates are worn out. Foam cushioning is not plate mounted or seamless. It is also a problem because the plate distorts. There are also problems with solvents and dimensional stability (which effects durability).
Dimensional stability is a requirement in rubber-covered flexographic printing operations since the outer surface of the rubber-covering must have a true cylindrical shape in order to perform the requisite printing functions, such as accurately imprinting an acceptable printing image onto a printing medium. This true cylindrical shape generally must be within a 0.0005"-0.002" tolerance level in order to be acceptable for flexographic printing applications.
From a commercial standpoint, if a cylinder is to be rubber-covered, it can be shipped both to and from a company who serves as the covering fabricator. Because of the weight and size of these cylinders, the cost of freight to and from the covering fabricator is quite high. Also, the time to complete the shipping and fabricating operation can be substantial and quite costly. The end user may have to undergo a loss of business because of the unavailability of a given rubber-covered cylinder at a time when a customer needs same for manufacturing purposes.
For the above reasons, and in order to reuse the plates, sleeves that fit over the plate cylinders can be used. These printing plate-printing sleeves assemblies can be mounted onto a plate cylinder using compressed air to expand the printing sleeve for conducting the mounting and dismounting operations. U.S. Pat. No. 4,903,597 (U.S. '597), for example, is directed to a composite laminate sleeve for the aforementioned purpose. U.S. '597, which is assigned to the assignee of this patent application, is incorporated herein by reference. However, the subject composite sleeve of U.S. '597 can only bridge two or three sizes of printing repeats. Furthermore, the U.S. '597 sleeve can become brittle when subjected to a vulcanizing process. Finally, since the U.S. '597 sleeve has a hard surface, a cushion is needed between the printing plate and the sleeve body.
The sleeve of U.S. Pat. No. 4,496,434 is fabricated of nickel. It can only be made in one thickness so it can't be used to increase repeat circumferences. Because of the 0.005" wall thickness, this sleeve bends easily and when it becomes bent it is no longer usable. Since the U.S. '434 sleeve is thin, it also can become damaged during the mounting operation. Since the U.S. '434 sleeve has a hard surface, it requires a cushion between the plate cylinder and the plate. Finally, the U.S. '434 sleeve exhibits a low level of durability.
U.S. Pat. No. 3,978,254 is directed to a carrier sleeve formed by three interlaminated layers, each layer formed of a helically-wound plastic tape. This sleeve comprises a single thickness, requires a cushion and has limited durability. It also can have storage problems.
U.S. Pat. Nos. 4,144,812 and 4,144,813 provide non-cylindrical printing sleeves and associated air-assisted printing rolls designed in a tapered or stepped-transition configuration, the change in the sleeve or printing cylinder diameter from one end to the other being progressive, i.e., increasing or decreasing according to the direction one is moving along the printing sleeve or roll. The printing roll comprises an outer surface having one end of a diameter greater than the other longitudinal end. The printing sleeve has an inner surface designed to form an interference fit with the outer surface of the printing roll only at the designated working position, and not along the entire axial uniform cross-sectional extent of the tapered sleeve. The '812 and '813 systems require a non-conventional, non-cylindrical plate cylinder, need higher than normal air pressures to operate, have only one thickness, dimension, and need cushioning between the cylinder and the plates.
Certain composite sleeves are available which deform if unsupported during vulcanization. Certain other sleeves, even if supported during the vulcanization process, become more brittle during vulcanization thereby reducing their durability. Finally, some sleeves are, due to the nature of the materials of construction, incapable of being vulcanized. Sleeve systems which exhibit problems such as storability, durability, inability to be vulcanized, a single thickness, the need of cushioning, or which have a total indicated run out which is not dependable, include the following: the Dupont Cyrel.RTM. Sleeve, the Miller Sleeve (Great Britain), the Saueressig Sleeve (Germany), the Rossini Sleeve (Spain), and the Roltec Sleeve (Germany).